For LTE EPC (Long Term Evolution Evolved packet Core) a Tracking Area, TA, corresponds to the Routing Area (RA) used in Wideband Code Division Multiple Access (WCDMA) and GSM/Edge Radio Access Network (GERAN). The TA consists of a cluster of RBSs having the same Tracking Area Code (TAC). The TA provides a way to track UE locations in idle mode. TA information is used by the Mobility Management Entity (MME) when paging idle UE's to notify them of incoming data connections.
If the UE moves out of the TA it must signal to the network and perform TA Update. This is done so the network can limit the area of which to page the UE in case of downlink data in idle mode. Otherwise the MME would have to page the entire network. Paging is generally seen as a resource demanding operation.
For LTE EPC systems the standards 3GPP TS23.401 and TS24.301 suggests the possibility for an MME to send a list of TA's during TA Update for which a UE may roam without signaling to the network. In LTE, the MME provides the UE with a list of tracking areas where the UE registration is valid. When the MME pages a UE, a paging message is sent to all radio base stations associated with the TA list.
First when the UE roams outside the TA area defined by the TA list, a TA update, TAU, is performed. A short TA list affects the UE negatively as it must perform TAU more often when moving. A large TA list will reduce the signaling based from the UE since it doesn't need to perform TA update so often and will therefore save power for the UE. However, a large TA list creates a higher signaling load for the network in case the UE needs to be paged since paging must be done in a larger area.
A trade-off in list length may be chosen. As a practical solution, a list length of e.g. 15 may be selected for LTE EPC systems. One possible implementation is to collect the trace of 15 recent visited TA's for a given UE, arrange those TA's in a list for the UE which then define the paging area.
The Tracking Area Update, TAU, procedure with Serving GW change is explained in 3GPP TS 23.401, 3GPP TS 23.401 V11.2.0 (2012-06), chapter 5.3.3.1, c.f. fig. FIG. 5.3.3.1-1: Tracking Area Update procedure with Serving GW change showing among other signal 3, TAU request from U to MME and signal 20 TAU Accept from MME to UE.